Nitric oxide regulation of the central aortae of the toad Bufo marinus occurs independently of the endothelium

Nitric oxide (NO) signalling pathways were examined in the lateral aortae and dorsal aorta of the cane toad Bufo marinus. NADPH diaphorase histochemistry and nitric oxide synthase (NOS) immunohistochemistry found no evidence for endothelial NOS in the endothelium of toad aortae, but it could be readily demonstrated in rat aorta that was used as a control. Immunohistochemistry using a specific neural NOS antibody showed the presence of neural NOS immunoreactivity in the perivascular nerves of the aortae. The anatomical data was supported by in vitro organ bath physiology, which demonstrated that the vasodilation mediated by applied acetylcholine (10-5 mol l-1) was not dependent on the presence of the vascular endothelium; however, it was significantly reduced in the presence of a neural NOS inhibitor, vinyl-L-NIO (10-4 mol l-1). In addition, atropine (10-6 mol l-1) (a muscarinic receptor inhibitor), L-NNA (10-4 mol l-1) (a NOS inhibitor) and ODQ (10-5 mol l-1) (an inhibitor of soluble guanylyl cyclase) abolished the vasodilatory effect of applied acetylcholine. In conclusion, we propose that an endothelial NO system is absent in toad aortae and that NO generated by neural NOS in perivascular nerves mediates vasodilation.